EssentialAction is the underlying functional type used by Play’s HTTP APIs. This differs from the Action type in Java, a higher-level type that accepts a Context and returns a CompletionStage<Result>. Most of the time you will not need to use EssentialAction directly in a Java application, but it can be useful when writing filters or interacting with other low-level Play APIs.

To understand EssentialAction we need to understand the Play architecture.

The core of Play is really small, surrounded by a fair amount of useful APIs, services and structure to make Web Programming tasks easier.

Basically, Play’s action API abstractly has the following type:

RequestHeader -> byte[] -> Result

The above takes the request header RequestHeader, then takes the request body as byte[] and produces a Result.

Now this type presumes putting request body entirely into memory (or disk), even if you only want to compute a value out of it, or better forward it to a storage service like Amazon S3.

We rather want to receive request body chunks as a stream and be able to process them progressively if necessary.

What we need to change is the second arrow to make it receive its input in chunks and eventually produce a result. There is a type that does exactly this, it is called Accumulator and takes two type parameters.

Accumulator<E,R> is a type of arrow that will take its input in chunks of type E and eventually return R. For our API we need an Accumulator that takes chunks of ByteString (essentially a more efficient wrapper for a byte array) and eventually return a Result. So we slightly modify the type to be:

RequestHeader -> Accumulator<ByteString, Result>

Ultimately, our Java type looks like:

Function<RequestHeader, Accumulator<ByteString, Result>>

And this should read as: Take the request headers, take chunks of ByteString which represent the request body and eventually return a Result. This exactly how the EssentialAction’s apply method is defined:

The Result type, on the other hand, can be abstractly thought of as the response headers and the body of the response:

Result(ResponseHeader header, ByteString body)

But, what if we want to send the response body progressively to the client without filling it entirely into memory? We need to improve our type. We need to replace the body type from a ByteString to something that produces chunks of ByteString.

We already have a type for this and is called Source<E, ?> which means that it is capable of producing chunks of E, in our case Source<ByteString, ?>:

Result(ResponseHeader header, Source<ByteString, ?> body)

If we don’t have to send the response progressively we still can send the entire body as a single chunk. In the actual API, Play supports different types of entities using the HttpEntity wrapper type, which supports streamed, chunked, and strict entities.

which reads as the following: Take the RequestHeader then take chunks of ByteString and return a response. A response consists of ResponseHeaders and a body which is chunks of values convertible to ByteString to be written to the socket represented in the Source<E, ?> type.